The information described in this background section is not admitted to be prior art.
The United States Environmental Protection Agency, as part of its regulatory mandate under the Clean Air Act, requires the monitoring of particulate emissions from stationary sources such as power plants, manufacturing plants, and the like, to ensure that such sources do not exceed emission limits. Particulate emissions can be monitored by measuring the opacity of gases as they flow through emission stacks or other conduits. In this regard, the measured opacity of gases can be used as an indirect indicator of the level of particulate matter emissions in the gases. The opacity of a gas is defined as the percentage of visible light attenuated due to absorption, reflection, and scattering by particulate matter entrained in the gas (i.e., transparent stack emissions that do not attenuate visible light will have a transmittance of 100 percent and an opacity of zero percent, whereas opaque stack emissions that attenuate all visible light will have a transmittance of zero percent and an opacity of 100 percent). The measurement of the transmittance/opacity of a gas is known as optical transmissometry.
The transmittance/opacity of a gas can be measured using an instrument called an opacity monitor (also known as a transmissometer). Opacity monitors correlate the measured opacity of a gas passing through an emission stack or other conduit to the mass concentration of particulates in the gas. Opacity is measured by projecting a light beam through the gas and determining the difference between the initial intensity of the light beam and the intensity of the light beam which strikes a sensor/detector after having passed through the gas. In single-pass opacity monitors, the sensor/detector is placed on a stack or other conduit wall opposite the light source. In double-pass opacity monitors, a retroreflector is located on the stack or other conduit wall opposite the light source and reflects the light back through the stack or other conduit to a detector positioned near the light source.